1. Field of the Invention
The present invention relates to an optical encoder which is used for displacement measurement or angle measurement and which is capable of detecting an absolute position.
2. Related Background Art
An optical encoder basically includes a main scale in which a first optical grating is formed, an index scale which is disposed so as to face the main scale and in which a second optical grating is formed, a light source for radiating light to the main scale, and a light receiving element which serves to receive light transmitted through or reflected by the first optical grating of the main scale to return from the second optical grating of the index scale.
A system using a light receiving element array serving as an index scale as well in an optical encoder of this sort has been already proposed in Japanese Patent Application Laid-Open No. H06-56304 for example. In addition, there is also filed Patent Application entitled “Optical Encoder” having the similar construction and laid open as Japanese Patent Application Laid-Open No. 2003-161645.
An encoder having this construction is called an encoder of an incremental type and can detect an amount of movement based on an increase or a decrease in pulses for the movement of a scale. The encoder of an incremental type involves a problem in that since an absolute position of a rotation angle is obscure, so a sensor for detecting the absolute position is specially required.
In Japanese Patent Application Laid-Open No. H10-318790, in order to avoid the above problem to detect the absolute position in a transmission type encoder utilizing an incremental system, as shown in FIG. 7, transmittances of slits 2 disposed in a scale 1 are changed. For example, the transmittance of the slit 2a is set to 1, and the transmittances of the slits 2b, 2c, and 2d arranged in a row and in order are gradually reduced.
FIG. 8 shows a change in signal obtained when portions in which the transmittances of the slits 2 are changed pass through a sensor in a case where the scale 1 is used. Thus, analog two phases signals Oa and Ob are obtained from the sensor. Amplitudes of the analog two phases signals Oa and Ob are reduced due to the gradual reduction of the transmittances of the slits 2. Thus, this change is detected, thereby detecting the absolute position.
However, in case of the absolute position detecting means of the encoder as described above, for a method of detecting the signal amplitude, it is required to carry out the sampling at a timing much shorter than one period of the resultant encoder signal and to obtain the voltages of a peak and a valley of the signal obtained therefrom.
Consequently, in a state in which the scale is stopped, it is impossible to know a position where a predetermined portion is located.
In addition, in order to know the amplitude of the signal, there is required a circuit, having a large circuit scale, such as a high-speed A/D converter for carrying out sampling at a short timing. Moreover, it is necessary to precisely form the portions, having different transmittances, such as the slits 2 of the scale 1 shown in FIG. 7. Thus, there is a possibility that the actual amplitude change disperses.